# std::is_union

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class T > struct is_union; |  | (since C++11)

`std::is_union` is a UnaryTypeTrait.

Checks whether `T` is a union type. Provides the member constant value, which is equal to true if `T` is a union type. Otherwise, value is equal to false.

If the program adds specializations for `std::is_union` or `std::is_union_v`, the behavior is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > constexpr bool is_union_v = is_union<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is a union type, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Example

Run this code

    #include <type_traits>

    struct A {};
    static_assert(!std::is_union_v<A>);

    typedef union
    {
        int a;
        float b;
    } B;
    static_assert(std::is_union_v<B>);

    struct C { B d; };
    static_assert(!std::is_union_v<C>);

    static_assert(!std::is_union_v<int>);

    int main() {}

### See also

is_class (C++11) | checks if a type is a non-union class type (class template)

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